The present invention relates to an air-fuel mixture supply device, and more particularly relates to an air-fuel mixture supply device for an internal combustion engine in which fuel is injected into an air stream upstream of a disk-shaped butterfly type throttle valve which is rotatably mounted in an intake passage.
In an air-fuel mixture supply device such as, for example, a carburetor, with such a throttle valve, when the opening of the throttle valve is relatively small, then small droplets of fuel which impinge on the upstream face of the throttle valve tend to stick thereon, to collect together, and to form larger droplets. As a result, fuel temporarily collects on the upstream face of the throttle valve, and then drips off this edge at random places and times, and may tend to trickle down the wall of the intake passage in liquid form without being properly vaporized. This means that the distribution of fuel to the various cylinders of the engine can be very poor, and they may receive air-fuel mixture of different air/fuel ratio, and this ratio may vary over time as well. As seen from the illustrative graph FIG. 1, which illustrates air/fuel ratio as delivered to the various cylinders of an internal combustion engine with a conventional throttle valve, this distribution, as stated above, can be very uneven, and this leads to poor fuel economy of the engine, poor performance with regard to emissions of pollutants, and uneven running and acceleration. The air/fuel ratios shown in FIG. 1 are measured with 4-cylinder, 2000 cc engine under engine operating conditions of 2,600 r.p.m. and 5 kg-m wherein T represents the average air/fuel ratio, and #1, #2, #3 and #4 represent the air/fuel ratios at the portions at which the exhaust manifold branches are joined.
Even if each cylinder of the engine has a separate air-fuel mixture supply device, these problems are not obviated, because of the variation of air/fuel mixture ratio with time. Therefore even in this case poor engine performance results.